I've converted a few if these to the beam60 gas unit. Its quite simple to do and you can retain your original carb/mixer.

Thanx for all the replies, The same customer has two, 2 year old Nissan's, and two older Toyota's, with impco cobra regulators, and they run fine on the same propane fuel?
The Toyota dealer says 6 to 12 months is normal life span for the Aisian regulators. I drain the Aisian regulators, once a month, about 150 hrs.
Also when I tear the bad regulator apart, I find oily tar thru out the inside of the regulator, and the rubber valves are either badly groved or broken up.

Just a little in site into LPG fuel.
1. LPG fuel is either Butane or Propane gas or a mixture of both.
2. Typically, Propane is used in places north of the Mason-Dixon line as it is a bit less like to freeze up when it gets "**** cold"- like maybe 10% less. Butane is commonly used below the Mason Dixon line where it is warmer and when it heats up above say 72 degree (STP or Standard Temperature & Pressure) there is less oxygen in the ambient air for complete combustion.
3. LP fuels do not use an Octane rating like gasoline, they use a thing called BTU (British Thermal Units). This can be a bit tricky to understand but don't blame me blame our "friend and former landlords" the British.
A BTU a measurement of heat created by burning (e.g combustion). Propane gas has a different rating than Butane and they are as listed below along with BTU values for gasoline Diesel & Natural gas. Note these are close approximations will and when the ambient temperature and barometric pressures rise above or above 70F & ~30 barometric pressure. This is old school (data from my Fuels & Lubricant text book in use in 1966) but it was new to me when I was younger.
Propane: 92,500 BTU/gallon or 2570 BTU/cu.ft
Butane: 102,600 BTU/gallon or 3260 BTU/cu.ft.
Gasoline: 135,00 BTU/gallon or N/A
Diesel: 139,200 BTU/gallon or N/A
Natural gas: N/A or 1075 BTU/cu.ft.
A nice conversion factor there are 4.24 lbs. of propane in one gallon.

Simply stated the higher the BTU value the more power per per cylinder /per power stroke of the engine.

Now I said all that it might be possible the tar build up issue could be related to the type of LP fuel a supplier is providing. In our capitalistic society it is not uncommon for suppliers to "short sheet" customers. In that they are providing a low cost fuel which I would think would be propane - less power might equal lower cost. But butane based LPG might be better suited for the. area and will vaporize faster & = less tar build up. Check it out at your or your customer's end.

The info on natural gas might give you some insight into why the CNG phenomenon for lift trucks didn't take off like a rocket ship. When compared to LP fuel NG has significant less BTU than either Propane or Butane. CNG powered truck are noticeably sluggish (aka less productive).

you can buy a drain valve kit at your local dealer. it's a good idea to keep them drained. if you find you have a lot of oil every week drain it frequently or change suppliers.

is the gas outlet at the bottom of the unit, it should be as this stops the build up in the gas unit as the oil is drawn into the engine

I would say that the propane is poor quality because I service Toyotas that go over 5 years and 15000 hours without touching the regulators.

Going to get buildup in any LP system no matter what make it is, can drain it on a regular basis will help some. Could also put an inline thermostat in the cooling hose coming out of the regulator that might help it some,some trucks come from the factory with this thermostat installed.